1. Field of the Invention
The present invention relates to a touch panel, and more particularly, to a readout circuit which reads charge quantity of the touch panel.
2. Description of Related Art
With the blooming development in the electronic technology, and the prevalence of wireless communication and the Internet, a variety of electronic devices are becoming indispensable in people's day-to-day life and work. However, it is rather difficult to operate the most common input-output (I/O) interface, such as a keyboard or a mouse. Compared with a keyboard and a mouse, a touch panel is a simpler I/o interface. Therefore, the touch panel is usually applied as a man-machine interface between a man and an electronic device so as to perform some control operation.
Generally speaking, the touch panel can be classified into a resistive touch panel, an optical touch panel, and a capacitive touch panel, etc. On the other hand, the touch panel can also be classified into a current-type touch panel and a charge-type touch panel when being classified in a readout manner. FIG. 1 is a schematic diagram of an optical charge-type touch panel and a traditional readout circuit. A touch panel 110 includes a plurality of data lines and a plurality of scan lines which are respectively driven by a source driver 120 and a gate driver 130. A plurality of sensor lines of the touch panel 110 are coupled to a readout circuit 140. Only one scan line, one data line, and one sensor line are shown in FIG. 1.
A storage capacitor Cst1 and a liquid crystal capacitor Clc are respectively coupled to a bias voltage VBIAS1 and a common voltage VCOM. The bias voltage VBIAS1 and a common voltage VCOM can be same or difference voltage(s). As the gate driver 130 turns on a switch SW1 via one of the scan lines, the source driver 120 correspondingly writes pixel data into the storage capacitor Cst1 and the liquid crystal capacitor Clc via one of the data lines. Since liquid crystal molecules in the liquid crystal capacitor Clc are correspondingly rotated due to a voltage difference between the pixel data and the common voltage VCOM, a corresponding gray-level of the pixel is displayed.
A photo transistor PT provides a discharging path between a storage capacitor Cst2 and a bias voltage VBIAS2 according to the bias voltage VBIAS2. Specifically, if a location of the photo transistor PT is not touched by a user, the photo transistor PT enhances a discharging speed of the storage capacitor Cst2 due to the external light irradiating. On the other hand, if the external light irradiating the photo transistor PT is diminished due to a user's touch, the photo transistor PT reduces the discharging speed of the storage capacitor Cst2. As the gate driver 130 turns on a readout switch SW2 via one of the scan lines, the readout circuit 140 reads the remained charge quantity of the storage capacitor Cst2 via one of the sensor lines, and simultaneously recharges the storage capacitor Cst2 to a normal rated voltage level.
The method that the readout circuit 140 detects the charge-type touch panel and determines the touch location mainly uses the inconsistent discharging of the storage capacitors Cst2 or whether a coupling capacitor exists or not. For the charge-type display panel 110, an integrator (i.e. an operational amplifier (OP-AMP) 141 and a feedback capacitor 142) is commonly disposed in the readout circuit 140 for detecting the charge difference of the touch panel 110. An analog-to-digital converter (ADC) 143 converts an integral result of the integrator into a corresponding digital code, and transmits the digital code to an image processing circuit 150, such that the image processing circuit 150 determines the touch location.
However, if the storage capacitor Cst2 or the coupling capacitor of the touch panel 110 is too large, a capacitance (area) of the feedback capacitor 142 must be increased in order to prevent an output saturation of the integrator. Furthermore, since each of the sensor lines needs one integrator, the area occupied by the readout circuit 140 is significantly large.